1. Field of the Invention
The present invention relates to a structure of a loudspeaker having a repulsion magnetic circuit, the loudspeaker being thin and suitable for reproducing low frequency sounds and reproducing sounds at large vibration amplitudes.
2. Related Background Art
The structure of a conventional loudspeaker having a repulsion magnetic circuit is shown in FIGS. 6 and 7.
There is a strong desire to make a vehicle door-mounted loudspeaker thin and light in weight, because doors of a vehicle have become thin in order to increase the inner space of the vehicle and because the space for mounting the loudspeaker has become small in order to mount other devices, such as an automatic door locking mechanism and an automatic window opening/closing mechanism in the door.
A small space in the door means a small space behind the diaphragm of a loudspeaker, resulting in a poor acoustic performance. As a result, sounds radiated from the loudspeaker, particularly low frequency sounds, are adversely affected.
In order to intensify low frequency sounds, it is therefore necessary to make a loudspeaker as thin as possible and the effective space in a door as large as possible.
Japanese Patent Laid-open publication No.1-98400 and the like teach that a repulsion magnetic circuit structure is effective in reducing the thickness of a loudspeaker, the structure having two magnets magnetized in the thickness direction and disposed with the same poles being faced each other.
As shown in FIGS. 6 and 7, according to this repulsion magnetic circuit structure, a flat diaphragm 150 is adhered to the upper end of a bobbin 141, and another diaphragm 152 is adhered to the lower end of the bobbin 141. The latter diaphragm 152 has an outer circumference area slanted and raised to the bottom surface of the flat diaphragm 150 where the diaphragms 150 and 152 are joined together.
The repulsion magnetic field type loudspeaker shown in FIG. 6 is structured such that a corrugation or edge extends radially from the outer circumference of the diaphragm 150. The diaphragm 150 and corrugation or edge are disposed above the bobbin 141 and a bobbin coil 140. A suspension, coil lead wires, input terminals 171, and the like are disposed under the bobbin 141.
The repulsion magnetic field type loudspeaker shown in FIG. 7 has a tipple structure of magnets 131, 132, and 135 constituting a magnetic circuit and a two-stage structure of upper and lower voice coils 140 and 143 to which diaphragms 150 and 151 are adhered. This loudspeaker is therefore thick.
In the case of the loudspeaker shown in FIG. 6, it is necessary to keep a space necessary for accommodating the diaphragm 150 and corrugation above the voice coil 140 and bobbin 141 in the sound radiation direction, as well as an additional space for permitting the vibration of the diaphragm 150. Furthermore, it is necessary to keep a space under the voice coil 140 and bobbin 141 on the back side of the loudspeaker, this space being necessary for accommodating the diaphragm 152 and the coil lead wires and input terminals under the diaphragm 152. The coil lead wires are required not to contact the diaphragm 152, a frame 120, and the like. From these reasons, the loudspeaker shown in FIG. 6 becomes thick.
The above-described structure also limits a vibration amplitude of the diaphragm 150, and an available space left in the door is small as described above. Therefore, low frequency sounds in particular are difficult to be reproduced from a loudspeaker, and a power performance (maximum allowable input power characteristics) is very low, as compared to another loudspeaker having the same diameter and used in a larger space.
In the case of the loudspeaker shown in FIG. 7, the tipple structure of the magnets 131, 132, and 135 and the two-stage structure of the voice coils 140 and 143 increase the thickness of the loudspeaker and reduce the space behind the loudspeaker. Furthermore, the outer circumferences of the diaphragms 150 and 151 are directly fixed to an outer frame 122 so that low frequency sounds are difficult to be reproduced more than the loudspeaker shown in FIG. 6 and the power performance is extremely low. This loudspeaker is low in practical value as a door-mounted loudspeaker.